1. Field of the Invention
The present invention relates generally to a receiving apparatus and method for a Subscriber Station (SS) in an Orthogonal Frequency Division Multiplexing (OFDM)-based broadband wireless communication system, and in particular, to an apparatus and method for cell acquisition and downlink synchronization acquisition in a Time Division Duplex-Orthogonal Frequency Division Multiple Access (TDD-OFDMA) communication system.
2. Description of the Related Art
In OFDMA communication systems based on the IEEE 802.16d/e standard, an SS identifies a cell and acquires synchronization to the cell using a pilot signal or a preamble signal received from a Base Station (BS). Any signal transmitted from the BS for assisting in cell acquisition and downlink synchronization in the SS is referred to as a “preamble signal”. Cell acquisition and synchronization acquisition in IEEE 802.16d/e based systems will be described.
FIG. 1 illustrates the configuration of an OFDMA-based broadband mobile communication system.
Referring to FIG. 1, the OFDMA communication system is configured to have a single cell structure. It is comprised of a BS 100 and a plurality of SSs 110, 120 and 130, each managed by the BS 100. Signal transmission/reception takes place in OFDM/OFDMA between the BS 100 and the SSs 110, 120 and 130. Thus, the SSs 110, 120 and 130 and the BS 100 transmit physical channel signals on subcarriers.
In the OFDMA communication system having the above-described configuration, upon power-on, an SS attempts to access a BS. The SS first acquires synchronization to receive information from the BS. The synchronization is the process of detecting the absolute time of a received signal and acquiring a signal in a desired time period among successively received signals. The synchronization is divided into primary rough synchronization referred to as frame synchronization, and secondary fine synchronization for acquiring accurate timing synchronization. For a wireless connection, the SS performs a cell acquisition in which it estimates information about a BS to transmit the most acceptable signal to the SS, after frame synchronization. Then the SS carries out fine synchronization with the BS to prevent degradation of a received signal from arising from asynchronization.
Once the SS is connected to the BS, the SS transmits/receives data to/from the BS. If the SS roams (i.e. the wave propagation time varies according to the moved distance) or the clock signal from the SS does not trigger and instead drifts, the SS loses the initially acquired downlink timing synchronization. To solve this out-of-synchronization problem, the SS prevents a received signal from being degraded through periodic synchronization (synchronization tracking) while connected to the BS.
Also, if the SS moves away from the BS after the connection, the Signal-to-Noise Ratio (SNR) of a signal received from the BS decreases. That is, as the SS is farther from the BS, path loss increases, causing handoff. The SS then attempts a handoff and tries to access a BS that offers the highest SNR. During this operation, the SS performs synchronization and neighbor cell acquisition. If the cell acquisition and synchronization take place after the handoff is decided, a long time delay occurs until the handoff is completed. The SS must acquire information about neighbor cells (SNR, timing offsets, and cell acquisition) beforehand in preparation for the handoff.
Now a description will be made of how the preamble signal is transmitted.
FIGS. 2A and 2B illustrate the format of a preamble signal used in a typical TDD-OFDMA system. Specifically, FIG. 2A illustrates the preamble signal in the frequency domain and FIG. 2B illustrates the preamble signal in the time domain.
Referring to FIG. 2A, a preamble code is allocated to even-numbered subcarriers. As a result, the preamble signal has a repeated pattern in the time domain as illustrated in FIG. 2B. The preamble code carried on the used subcarriers is unique according to a cell ID. Therefore, correct detection of the preamble code leads to acquisition of the cell ID. Table 1 below summarizes the characteristics of preamble signals proposed in the IEEE 802.16d standards.
TABLE 1WiBroIEEE 802.16dNon-zero pilot2 subcarriers3 subcarriersspacingNumber of432577non-zero pilotsLength in time2 OFDMA symbols1 OFDMA symbolPAPR4.87~6.794.21~5.14Number of1016114modulation sequence(127 cell IDs ×8 sectors)Tx power boosting3 dB9 dBper pilot(identical OFDMA)(4.26 dB higher OFDMA)
An apparatus for transmitting a preamble with the above characteristics will be described.
FIG. 3 is a block diagram of a preamble transmitter in the typical TDD-OFDMA communication system.
Referring to FIG. 3, a preamble code generator 301 generates a cell-specific preamble code using received cell information (IDcell, s). A preamble channel generator 303 allocates the preamble code to subcarriers, that is, allocates each element or bit of the preamble code to a corresponding input (subcarrier position) of an Inverse Fast Fourier Transform (IFFT) processor 305. As noted from Table 1, in a WiBro system, the preamble channel generator 303 allocates the preamble code to even-numbered subcarriers and pads null (zeroes) at the remaining odd-numbered subcarriers. The IFFT processor 305 generates a time-domain signal by IFFT-processing the signal received from the preamble channel generator 303. A parallel-to-serial converter 307 converts the parallel IFFT data to serial data. A Cyclic Prefix (CP) inserter 309 inserts a CP into the serial data stream, thereby generating a baseband preamble signal. While not shown, the baseband preamble signal is processed into a Radio Frequency (RF) signal transmittable over the air and then transmitted over the air through an antenna.
Initial cell search using the preamble signal will be described below.
FIG. 4 is a block diagram of a cell search apparatus for an SS in a conventional TDD-OFDMA communication system.
Referring to FIG. 4, a serial-to-parallel (S/P) converter 401 parallelizes a received preamble signal (OFDM symbol). A Fast Fourier Transform (FFT) processor 403 FFT-processes the parallel signals and outputs a frequency-domain FFT signal. A differential correlator 405 correlates the FFT signal with all possible preamble codes. To be more specific, the differential correlator 405 multiplies the FFT signal by a preamble code, correlates adjacent subcarriers in the product, sums the correlations, and outputs the sum as a final correlation for a corresponding cell. For example, if there are 127 cell IDs and 8 cells are given per cell, the differential correlator 405 searches 127×8 cells and outputs 127×8 correlations, after power-on in the SS. A maximum value detector 407 selects a preamble code (or cell ID information) with the highest correlation. Since the SS does not have a prior knowledge of a cell at an initial access, it must perform an exhaustive search over all cases to acquire information about a cell in which it is located.
The above-described cell search is modeled as Equation (1):
                                          {                          IDcell              ,              s                        }                    =                      arg            ⁢                                                  ⁢                                          max                                                      0                    ≤                    IDcell                    <                    126                                    ,                                      0                    ≤                    s                    <                    8                                                              ⁢                              {                                                                        f                                          IDcell                      ,                      s                                                                                        }                                                    ⁢                                  ⁢                              f                          IDcell              ,              s                                =                                    ∑                              j                =                0                                            W                -                2                                      ⁢                                                  ⁢                                                            Y                                      IDcell                    ,                    s                                                  ⁡                                  (                  j                  )                                            ·                                                Y                                      IDcell                    ,                    s                                    *                                ⁡                                  (                                      j                    +                    1                                    )                                                                                        (        1        )            where fIDcell,s denotes an auto-correlation for a cell {IDcell,s}, W denotes the size of an input signal used for auto-correlation, and YIDcell,s(j) denotes the received signal response of a jth subcarrier among the subcarriers that carry the preamble, after the FFT processing and the preamble code multiplication.
After the cell acquisition, the timing offset of the cell is detected.
The timing offset detection is performed by Equation (2):
                    τ        =                  arg          ⁢                                          ⁢                                    max                                                t                  min                                ≤                n                ≤                                  t                  max                                                      ⁢                                                                          ∑                                      j                    =                    0                                                        W                    -                    1                                                  ⁢                                                                  ⁢                                                                            Y                                              IDcell                        ,                        s                                                              ⁡                                          (                      j                      )                                                        ·                                      ⅇ                                                                  -                        j2                                            ⁢                                                                                          ⁢                      π                      ⁢                                                                                          ⁢                                              f                        ⁡                                                  (                          j                          )                                                                    ⁢                                              n                        /                                                  N                          FFT                                                                                                                                                                                          (        2        )            where W denotes the total number of subcarriers that carry the preamble signal, f(j) denotes the actual frequency index of the jth subcarrier among the preamble subcarriers, and YIDcell,s(j) denotes the received signal response of the jth subcarrier among the preamble subcarriers, after FFT processing and preamble code multiplication.
Referring to Equation (2), YIDcell,s(j) is a signal input to a timing offset detector. The timing offset detector multiplies the input signal by an exponential function. A variable included in the exponential function is n ranging [tmin˜tmax] Thus, Equation (2) is computed over all possible values of n and a particular n that maximizes Equation (2) is selected as a timing offset estimate. [tmin˜tmax] is a system operation parameter.
Acquisition of handoff information using the preamble signal will be described now.
FIG. 5 is a block diagram of a neighbor cell acquisition apparatus for the SS in the conventional TDD-OFDMA communication system.
Referring to FIG. 5, an S/P converter 501 parallelizes a received preamble signal (OFDM symbol). An FFT processor 503 FFT-processes the parallel signals and outputs a frequency-domain FFT signal. Typically, a home BS (i.e. serving BS) broadcasts cell information about neighbor BSs to all SSs. Therefore, a differential correlator 505 correlates the FFT signal with each of the known preamble codes of neighbor cells and outputs a plurality of correlations. To be more specific, the differential correlator 505 primarily multiplies the FFT signal by a preamble code, correlates adjacent subcarriers in the product, sums the correlations, and outputs the sum as a final correlation for a corresponding cell. That is, the differential correlator 505 outputs |fIDcell,s| of Equation (1) as a correlation. An arranger 507 arranges the plurality of correlations in a descending order and thus prioritizes the neighbor cells as target cells for handoff.
The above-described neighbor cell search is modeled as Equation (3):{IDcell,s}=sort{|fIDcell,s|}  (3)
The timing offset of the acquired neighbor cells are detected according to their priority levels by Equation (4):
                              τ                      IDcell            ,            s                          =                  arg          ⁢                                          ⁢                                    max                                                t                  min                                ≤                n                ≤                                  t                  max                                                      ⁢                                                                          ∑                                      j                    =                    0                                                        W                    -                    1                                                  ⁢                                                                  ⁢                                                      Y                                          IDcell                      ,                      s                                                        ·                                      ⅇ                                                                  -                        j2                                            ⁢                                                                                          ⁢                      π                      ⁢                                                                                          ⁢                                              f                        ⁡                                                  (                          j                          )                                                                    ⁢                                              n                        /                                                  N                          FFT                                                                                                                                                                                          (        4        )            
The parameters in Equation (4) and Equation (2) are alike in their definitions.
How synchronization to a home cell is tracked using the preamble signal will be described.
FIG. 6 is a block diagram of a synchronization tracking apparatus for the SS in the conventional TDD-OFDMA communication system.
Referring to FIG. 6, an S/P converter 601 parallelizes a received preamble signal (OFDM symbol). An FFT processor 603 FFT-processes the parallel signals and outputs a frequency-domain FFT signal. A multiplier 605 multiplies the FFT signal by the known preamble code of the home cell. A phase detector 607 multiplies the product received from the multiplier 605 by each of a plurality of exponential functions determined according to a predetermined search range. A maximum value detector 609 selects a signal with the highest value among the output signals of the phase detector 607, and thus acquires the timing offset of the home cell.
The above-described synchronization tracking for the home cell is modeled as Equation (5):
                    τ        =                  arg          ⁢                                          ⁢                                    max                                                t                  min                                ≤                n                ≤                                  t                  max                                                      ⁢                                                                          ∑                                      j                    =                    0                                                        W                    -                    1                                                  ⁢                                                                  ⁢                                                                            Y                                              IDcell                        ,                        s                                                              ⁡                                          (                      j                      )                                                        ·                                      ⅇ                                                                  -                        j2                                            ⁢                                                                                          ⁢                      π                      ⁢                                                                                          ⁢                                              f                        ⁡                                                  (                          j                          )                                                                    ⁢                                              n                        /                                                  N                          FFT                                                                                                                                                                                          (        5        )            
The parameters shown in Equation (5) and Equation (2) are alike in their definitions.